Concept:Mosquitoes breed only in stagnant or still water, not in flowing water.
Explanation:Female mosquitoes lay their eggs on the surface of calm water.
The eggs hatch into larvae and pupae which live in the water.
These immature stages need to breathe air from the surface using breathing tubes.
A river has fast-moving, constantly flowing water.
The current washes away eggs, disrupts larvae, and prevents them from staying at the surface to breathe.
Therefore, a river is an unsuitable breeding habitat for mosquitoes.
On the other hand, pools, ponds, and swamps often hold still or slow-moving water.
These calm water bodies provide a suitable environment for mosquito breeding.
Answer:D. river.